Researchers from the Smithsonian Institution traveled to a remote location in northern Canada to collect samples of four-billion-year-old rocks that may reveal secrets about Earth's early history.
All humans know that the Earth is quite old. Scientists estimate that
the planet formed about 4.5 billion years ago. But how old is it? And what was it like when planet Earth was still forming?
These questions have long preoccupied researchers from all over the world, searching for clues on different continents. But because most of the rocks have been lost to time, it is rare to find physical remnants of the first few hundred million years of the planet's existence. Last summer, geologists from the Smithsonian Institution set out to fill in those gaps.
First steps and findings
Michael Ackerson, a research geologist at the Smithsonian's National Museum of Natural History, led an expedition to Canada's Northwest Territories to collect samples from the Acasta Gneiss, a remote outcrop that contains the oldest known rocks
on Earth. Some of them date back more than four billion years.
Akerson and his colleague Vriju Chowdhury, who is doing postdoctoral research, brought the samples to Washington, D.C., where they are being studied and stored in the museum's National Rock and Ore Collection.
The trip was part of an initiative called Our Unique Planet, which brings together researchers from different disciplines to explore the fundamental question of what makes Earth so different from all the other known planets in the solar system.
"Earth is a unique planet in three senses," says Ackerson. "It has continental crust, oceans, and life. No other planetary body has all three of these qualities."
To understand how these features came to be, researchers must look into the past - far back in time. "What was the Earth like four billion years ago?," Ackerson asks. "How can this help us understand the evolution of our planet?".
Photo: theconversation.com
The gray gneisses of the Acasta region are useful in this matter. Last year, Chowdhury said that it was only in 1999 that scientists confirmed the age of some of the rocks in this formation to be more than four billion years old, the oldest known preserved crustal material on the planet. "This is a snapshot of the first 500 million years," Chowdhury says. "It's like looking at the planet through a pinhole."
The gneisses of the Acasta region
According to Ackerson, the rocks he mined were collected from a surprisingly small and fragmented area - about the size of the red zone of a soccer field. The ancient material, scattered in thin, discontinuous bands over an area of several kilometers, is difficult to identify and even harder to obtain.
Complicating things is the appearance of the Acasta area rocks. "They're all gray," says Jessie Reimink, a geologist at Pennsylvania State University who did his doctoral dissertation on Acasta gneiss and joined the trip last summer. "One gray rock can be completely different from another, even if they look the same."
Reimink acted as a field guide for the team, helping them distinguish the elusive ancient rocks from the surrounding terrain. "These rocks are unimaginably hard to find," says Ackerson. "You can stand with one foot on a 3.6-billion-year-old rock and the other foot on a four-billion-year-old rock."
To identify the oldest material, the researchers enlisted the help of a reliable tool: a magnet. "The oldest rocks, which are 4.02 to 4.03 billion years old, are magnetic," Ackerson says. "It's very convenient and comfortable."
They also looked for special minerals such as
granate, a dark red crystal found in the oldest rocks, and distinguished younger material by its slightly pinker hue. A combination of
magnetism, mineral clues and color differences helped them navigate the complex geological site.
They were looking for a type of metamorphic rock called gneiss. It forms when igneous rocks like granite are subjected to intense heat and pressure, changing their structure and appearance over time. But buried inside this ancient gneiss is something even more valuable: zircon.
The valuable mineral zircon
"Circones are resistant to pressure, temperature, erosion," Chowdhury explains. "They capture the chemical environment in which they were created." These tiny crystals form in magma and absorb trace amounts of uranium, which eventually decays into lead. By measuring the ratio of uranium to lead isotopes, scientists can determine the age of a crystal with surprising accuracy.
Ackerson compares this study to baking. "If you imagine the Earth today is like a beautiful birthday cake, looking at the final product, it's impossible to understand how it was made," he says. "We're trying to develop a reverse recipe, starting with the ingredients."
Unlocking the secrets of these ancient minerals could turn scientists' understanding of how Earth evolved from a molten ball of rock to a habitable planet upside down.
The first few hundred million years of Earth's history remain one of the most poorly understood chapters of geologic time, yet it was during this period that the foundations of modern plate tectonics, continental crust, and perhaps even early microbial life may have originated.
According to Akerson, understanding what conditions gave rise to the Earth that humanity knows today, and to life itself, begins with these ancient rocks. The samples from the Acasta area represent one of the only direct records from that era, providing a rare and important glimpse into the forces that shaped the world.
During the journey, the team collected more than 450 kilograms of rock. Getting the samples home was no easy task. They were brought from remote islands in inflatable boats and propeller planes in buckets, which were then transported in seaplanes and trains bound for Washington, DC.
Mineral research
In the lab, the samples undergo a rigorous preparation process. First, the team crushes the rock to fine grains and separates the zircons using thick liquids and magnetic fields. The researchers then measure the isotopic composition of the crystals, a process that requires careful calibration and precision. Each dated zircon provides a glimpse into the state of the Earth's crust at the time of its formation.
Photo: theconversation.com
Although the stones are already being analyzed for age and chemical composition, their story is just beginning. "What Mike and Vriju are doing is very creative," Reimink says. "They are looking for older material, tiny mineral grains or previously unexplored rocks that can further expand our understanding of the early Earth."
These samples, part of the National Collection of Rocks and Ores, will be available to scientists around the world. For the general public, this research will help expand the museum's understanding of Earth's history.
The Deep Past exhibition currently covers much of Earth's ancient past. "We hope to create a 'companion exhibition' that goes even further back," Ackerson says. "Trace history from today to the formation of our solar system."
This exhibit is still years away, but its foundation is already taking shape - one gray stone at a time. "These specimens contain clues about the earliest stages of our planet's evolution," Reimink says. "And more broadly, they help answer the fundamental question, Why are we here."